Quadrupole mass spectrometers are known in the art and typically operate as narrow band pass filters by appropriate selection and application of radiofrequency (RF) and direct current (DC) voltages to the quadrupole electrodes that correspond to the Mathieu a and q values near the apex of the first stability region. Quadrupoles typically comprises two pairs of cylindrical (preferably hyperbolic) rods that are arranged symmetrically about a central axis and oriented to receive ions that enter from one end. Ions that exit from the other end may be detected or further manipulated. A and q are obtained from the known equations
      q    =                  4        ⁢        e        ⁢                                  ⁢        V                              r          0          2                ⁢                  Ω          2                ⁢        m                  a    =                  8        ⁢        eU                              r          0          2                ⁢                  Ω          2                ⁢        m            
Where U is the DC voltage, V is the RF Voltage, r0 is the radius of the inscribed circle between the rods, Ω is the angular frequency (radians/second) of the drive voltage and m is the mass of the ion.
Quadrupoles can also operate in RF-only mode, commonly referred to as transmission mode RF-only mass spectrometers in which no resolving DC voltage is applied to the quadrupole electrodes as discussed for example in U.S. Pat. No. 4,090,075, incorporated herein by reference. Such RF-only mass spectrometers are known to provide unit resolution mass spectral peaks with poor quality quadrupoles [J. W. Hager, Rapid Communications in Mass Spectrometry, 13, 740(1999), herein incorporated by reference]. This state of operation corresponds to that where the Mathieu a-parameter is set to 0 and the quadrupole operates as a broad band, high pass filter. As the RF voltage is increased, near 100% transmission of an ion of particular mass is observed until the high Mathieu q-parameter cutoff at 0.907 is reached. At this point, ions become unstable and gain significant radial amplitude until they are removed by either contacting the electrodes or being ejected. At the exit of quadrupole devices, fringing fields are present that can convert radial energy of ions into axial energy. Accordingly, ions having large radial displacements within the fringing fields receive a proportionately greater kinetic energy boost from this conversion than those with small radial displacements. A downstream repulsive DC or AC barrier can be used to discriminate between the kinetic energy of the radially excited ions from the ions that have not been radially excited.
Transmission mode RF-only mass spectrometers have multiplexing advantages over conventional RF/DC quadrupole filters since ions at multiple m/z values can be transmitted simultaneously at unit resolution. This yields a multiplexing advantage to the extent that the same signal-to-noise in the RF-only device can be achieved which can increase the duty cycle of an instrument.
In addition to radial excitation that comes from operation at or near a stability boundary, such as that at a=0, q=0.907, radial excitation can also occur through interaction with an auxiliary AC field as described in U.S. Pat. No. 6,114,691, herein incorporated by reference. However, it is important that the background ion signal be discriminated from the radially excited ion signal to generate acceptable signal-to-noise in these transmission RF-only quadrupole mass spectrometers. Further background is described in U.S. Pat. No. 5,998,787, U.S. Pat. No. 6,028,308 and U.S. Pat. No. 6,194,717, herein incorporated by reference.
One of the difficulties with transmission mode RF-only mass selection is the problem of discrimination against background signal due to the presence of high-energy ions. This can be alleviated by the addition of an auxiliary excitation which can aid in imparting higher kinetic energies to ions that are resonant with the excitation frequency and increase the amount of associated kinetic energy relative to the background ion signal. However, this introduces a problem of distinguishing between the signal from a particular ion that would occur when in resonance with the auxiliary AC field and signal that arises from the usage at other instability boundaries, such as a=0, q=0.907. An example of this is shown in FIG. 4, in which the product ion spectrum of the protonated reserpine ion having an m/z ratio of 609 Da has been obtained in a tandem quadrupole mass spectrometer. In this setup, Q1 was operated as a standard RF/DC quadrupole in low resolution mode selecting the m/z 609 Da precursor ion and Q3 as an RF-only quadrupole mass spectrometer with auxiliary dipolar excitation at 616 kHz (drive RF=1.5 Mhz) where Q3 was scanned at 667 amu/sec. The upper spectrum of FIG. 4 shows the Q3 output of the tandem mass spectrometer over a broad range, the middle trace shows an expansion of the parent ion region and the bottom spectrum expands the vertical scale to show an additional feature at a m/z of 635 Da. The two RF-only signals in the bottom spectrum are both from residual precursor ions: one at m/z 609 which originates from resonance excitation using a 616 kHz auxiliary AC field and one that is present at m/z 635 derived from the a=0, q=0.907 instability boundary. The use of the transmission-mode RF-only quadrupole with auxiliary excitation therefore presents the possibility of contribution to the detected signal from ions at slightly smaller m/z values as well as excitation at the a=0, q=0.907 stability boundary. This reduces selectivity and signal-to-noise of the device. One can eliminate this difficulty in scanning mode by simply removing the auxiliary excitation and obtaining the mass spectrum at the a=0, q=0.907 stability boundary. However, this also eliminates the possibility of multiplexing in Selected Ion Monitoring (SIM) or Multiple Reaction Monitoring (MRM) modes, since for the use of these transmission-mode RF-only quadrupoles, there will always be a contribution to the transmitted ion signal from lower m/z ions at the a=0, q=0.907 stability boundary.